Method for recovery of metals from spent iron-chrome shift catalyst and method of forming new catalyst

ABSTRACT

The Abstract of the Invention is a method of making a solution of the nitrates of iron and chromium from a spent catalyst containing oxides of iron and chromium, which method comprises: 
     (a) dissolving the catalyst in sulphuric acid; 
     (b) reacting the resulting sulphates solution with a source of nitrate ions and of cations of which the sulphate is of low solubility, whereby to precipitate such sulphate; 
     (c) separating the sulphate, whereby to give a nitrates solution of low sulphate content.

This invention relates to solution preparation, in particular of thenitrates of iron and chromium, the production of a catalyst containingiron oxide and chromium oxide from such a solution, and to an oxidecomposition reducible to a shift catalyst.

The iron-chrome shift catalyst when in use contains a mixture ofdivalent and trivalent iron oxides, in combination with trivalentchromium oxide, and is usually brought into its active form by reducingan oxide precursor composition in a higher valency state. Some oxidecompositions used in the past have contained hexavalent chromium, butthis is undesirable because of toxicity and of the large evolutionduring reduction. It has, therefore, been proposed to reduce thehexavalent chromium to trivalent chromium during production of the oxidecomposition, for example, by reaction with a divalent iron compound suchas ferrous carbonate. This has the drawback that the only inexpensiveiron compound from which the ferrous carbonate could be made is ferroussulphate, so that steps have to be taken to avoid contamination of theoxide composition by sulfur or to prolong the reduction treatment untilsulfur compounds have been driven out. Among the methods proposed formaking low-sulfur oxide compositions are that described in UK Pat. No.1,259,381, which describes an initial stage of precipitation of ferrousoxalate, and that described in UK Pat. No. 1,252,176, which describesthermal decomposition of nitrates. These methods are expensive,especially that of UK Pat. No. 1,252,176 because of the cost of thechromium nitrate and, accordingly, in our European published application19989 (U.S. Ser. No. 131,390), we describe a method of making a solutionof the nitrates of iron and chromium by reacting metallic iron or analloy thereof with nitric acid in the presence of one or more hexavalentchromium compounds. Thereafter, the nitrates are converted to anintimate mixture of the oxides, preferably by way of co-precipitatingthem with an alkaline reactant.

Since iron-chrome shift catalyst has to be replaced after a few years'use, it would be very convenient if it could be converted to the mixednitrate solution and, thus, to fresh catalyst by the method described inour co-pending application. However, we find that spent iron-chromeshift catalyst, at least when so made, does not dissolve in nitric acidat a practicable rate. This is contrary to what is disclosed in USSRPat. No. 471112.

We have now discovered that the nitrates solution can be made from spentcatalyst by the steps:

(a) dissolving the catalyst in sulphuric acid;

(b) reacting the resulting sulphates solution with a source of nitrateions and of cations of which the sulphate is of low solubility, wherebyto precipitate such sulphate;

(c) separating the sulphate, whereby to give a nitrates solution of lowsulphate content.

For Step (a) the quantity of sulphuric acid is preferably sufficient todissolve the catalyst completely, in order to avoid changing the ratioof iron to chromium. Incomplete dissolution may, on occasion, have to beaccepted; however, for example, when the catalyst has sufferedaccidental overheating or has been used at unusually high temperatures,as in reducing gas production. In any event, however, it is advisable tocheck and, if necessary, adjust the iron to chromium ratio of thesolution. The concentration of the sulphuric acid is conveniently in therange 10-80, especially 20-40% w/w, at the start of dissolution.

To facilitate dissolution the spent catalyst is one that has, in thecourse of discharge by the user, been cooled carefully, for example, insteam-diluted air or in a strong current of ambient air or otherwise, soas to avoid increasing its temperature above 300° C. By such treatment,the catalyst is kept at approximately the Fe₃ O₄ oxidation level andsintering is avoided.

If desired, further iron and/or chromium can be introduced into thesolution. For example, an iron salt can be added, such as a sulphate. Ifexcess sulphuric acid has been used, it can be conveniently neutralizedwith ferrous carbonate or reacted with metallic iron: metallic iron mayassist by reducing trivalent iron oxide. To balance the added iron, anaddition of a chromium compound can be made, preferably in hexavalentform to oxidize ferrous iron to ferric iron. Other steps, such asair-blowing, can be applied at this stage to effect the oxidation. Ifmetallic iron is dissolved, suitable precautions should be taken to dealsafely with any evolved hydrogen.

Step (b) is preferably carried out by double decomposition of thesulphates solution with calcium nitrate. This reactant can be madeseparately or in situ mixing nitric acid with the sulphates solution andadding a calcium compound (conveniently the hydroxide or carbonate) tothe mixture. The resulting calcium sulphate precipitate is separated andwashed and can then be disposed of to a plaster manufacturer or aslandfill. The resulting nitrates solution is saturated with calciumsulphate, but the solubility thereof is low (under 0.25 g per 100 ml,with only slight variation with temperature) and may be too little tocontaminate the catalyst to be made. If more complete sulphate removalis required, the solution can be reacted with a cation forming aless-soluble sulphate, for example, the salts of strontium, barium orlead or an anion exchange resin.

In order to limit the sulphate content of the solution further, thesulphuric acid dissolution can be applied not to the whole of the spentcatalyst but to that part of it which is not soluble or not dissolved inconvenient conditions by nitric acid. Thus, the invention provides amethod of making a solution of the nitrates of iron and chromium from aspent iron-chrome shift catalyst by the steps of:

(a) dissolving the catalyst incompletely in nitric acid;

(b) separating the resulting solution from the undissolved residue;

(c) dissolving the residue in sulphuric acid;

(d) reacting the resulting sulphates solution with a source of nitrateions and of cations of which the sulphate is of low solubility, wherebyto precipitate such sulphate;

(e) separating the sulphate, whereby to give a nitrates solution of lowsulphate content.

Conveniently, the solutions produced in Steps (b) and (e) can then bemixed. If Step (a) proceeds to the extent of over 80% as is readilypossible using the process of our U.S. application, Ser. No. 380,980,filed May 24, 1982, then the sulphate content of the mixture can be verylow. Moreover, it can be economic to use in Step (d) a reagent producinga sulphate less soluble than calcium sulphate.

The spent catalyst may contain various impurities deposted on it duringuse. If these include alkali metal compounds, these dissolve in thesulphuric acid, will remain in the nitrates solution and will bediscarded with the filtrate after the co-precipitation. If theimpurities include silica, this will be removed with the precipitatedsulphate. If the spent catalyst contains sulfur, this will be expelledas H₂ S during dissolution in sulphuric acid.

The details of the catalyst and the subsequent steps in its productionare as described in our co-pending application, the relevant passagesfrom which are now reproduced.

Usually, such a catalyst is used as a fixed bed, hence the method theninvolves the further step of forming the oxides into pieces, which forthis type of catalyst suitably have all their dimensions in the range 2to 20 mm, with no dimension of a single piece more than 5 times another.The pieces are made suitably by compression of the oxide compositionwith a lubricant, such as graphite, into cylinders having flat,indented, or protrusioned end surfaces. Alternatively, they can be madeby extrusion or granulation, of a wet composition, possibly with abinder material. To give an active catalyst, the oxide composition isreduced by means of a mixture of steam and hydrogen or a compoundreactible with steam to produce hydrogen in the conditions of reduction,which usually include a temperature in the range 150°-400° C.

The ratio of iron to chromium in the catalyst is typically in the range20:1 to 6:1 calculated by weight as the sesquioxides. Usually this ratiois attained by controlling the weights of chromium compound and nitricacid used in the original preparation step. Perferably at least 50% ofthe nitrates are derived from spent catalyst. If a nitrates solutionratio adjustment is made, it is more convenient by adding iron nitrateto a solution containing too much chromium, since externally suppliedchromium nitrate is very expensive. A small addition of hexavalentchromium (for example up to 1% of the Cr₂ O₃ used) may be tolerable ifmade after the wet stages of catalyst preparation but is best avoided onaccount of its toxicity and its high reduction exotherm.

If desired, a lead salt may be added to the nitrates in order to takeadvantage of the effect of lead oxide in the low sulfur catalyst to beproduced by the method.

Conversion of the nitrates to an intimate mixture of oxides can besimply by evaporation and calcination if any hexavalent chromiumcompound has been a chromic acid or anhydride or an ammonium chromate,or if alkali can be tolerated in the oxide composition. However, it ispreferred to convert the nitrates to oxides by way of co-precipitatingthem with an alkaline reactant, such as a carbonate or hydroxide of analkali metal or ammonium. If desired, they can be co-precipitated asoxalates, and possibly thereafter converted to hydroxides; but this isnot necessary. The resulting hydroxides or oxalates are then thermallydecomposed to oxides. Co-precipitation with an alkali metal compound isnormally followed by thorough washing of the precipitate to decrease itsalkali content to under 0.4%, especially under 0.3% w/w calculated asequivalent Na₂ O.

By the method according to the invention, it is possible to make anoxide composition reducible to a catalyst for the reaction of carbonmonoxide with steam to form carbon dioxide and hydrogen, the compositionhaving the following properties:

Fe₂ O₃ /Cr₂ O₃ weight ratio in the range 20:1 to 6:1, especially 12:1 to8:1;

surface area 100-250, especially 120-200 m² /g;

mean pore radius 10-100, especially 15-50, Angstrom units;

loss on ignition at 900° C. over 10, especially 15 to 25% w/w.

The shift reaction over the catalyst made from the oxide composition byreduction is of the high temperature type, that is, at an outlettemperature between 350° and 500° C. The inlet temperature is typicallyin the range 300°-400° C.

EXAMPLE

Solution Preparation

Sulphuric acid 96.5% w/w (150 ml) was diluted with water (225 ml) andwarmed to 225° F. (107° C.). Then 100 g of spent iron-chrome hightemperature shift catalyst of the type described in our U.S.application, Ser. No. 131,390, and containing 84.7% w/w of iron oxide(calculated as Fe₂ O₃), 7.71% w/w of Cr₂ O₃, was added in powder form.The slurry was digested at 225° F. under refluxing conditions for twohours. As a result, complete dissolution was obtained. The solution wasdiluted with cold water to 700 ml. The resulting solution contained 23.6grams/liter of Fe₂ O₃, 2.3 grams/liter as Cr₂ O₃ and 51.1 grams/liter assulfate. Into it was stirred a solution of calcuim nitrate (442 ganhydrous) in water. The resulting solution was filtered to removecalcium sulfate dihydrate, and the resulting filtrate was adjusted to avolume of 2.0 l. The resulting filtrate was then used to prepare a freshshift catalyst and is employed in the Example titled "CatalystPreparation" below.

Catalyst Preparation

A solution at 50° C. of the ferric nitrate and chromium nitrate solutionfrom the above preparation was fed to a stirred solution of sodiumcarbonate (250 g per liter) at 60° C. until the pH was in the range7.0-8.5 measured at 25° C. The resulting suspension of the hydroxides ofiron and chromium was stirred until no further carbon dioxide wasevolved; then washed and dried. The dry oxide composition was found byanalysis to contain 68.4% by weight Fe₂ O₃, 7.42% Cr₂ O₃, 17.2% CaO,1.87% of sulfur expressed as SO₃, and 0.23% of sodium expressed as Na₂O.

Sulfur content of the resultant oxide composition could have beenminimized by treating the starting nitrate solution with barrium nitratefollowed by filtering off precipitated insoluble barrium sulfate.

A sample of the oxide composition was milled, mixed with 3% of itsweight of graphite, granulated and then compressed into 3.6×5.4 mm squatcylindrical pellets. These pellets are the catalyst precursor as handledin commerce. A sample of pellets was reduced to active form by means ofa steam-hydrogen mixture at 300° C. and then tested in the shiftreaction. The effluent from the reduction was sulfur-free by the end ofreduction. The catalyst had activity in the shift reaction at leastequal to that of a commercially available high temperature shiftcatalyst.

Having thus described our invention, we claim:
 1. A method of making asolution of the nitrates of iron and chromium from a spent catalystcontaining oxides of iron and chromium, which method comprises:(a)dissolving the catalyst in sulphuric acid; (b) reacting the resultingsulphates solution with a source of nitrate ions and of cations of whichthe sulphate is of low solubility, whereby to precipitate such sulphate;(c) separating the sulphate, whereby to give a nitrates solution of lowsulphate content.
 2. A method of making a solution of the nitrates ofiron and chromium from a spent catalyst containing oxides of iron andchromium which method comprises:(a) dissolving the spent catalystincompletely in nitric acid; (b) separating the resulting solution ofthe nitrates of iron and chromium from the nitric acid insoluble residuecontaining undissolved iron and chromium components; (c) dissolving thenitric acid insoluble residue in sulphuric acid to obtain a solution ofmetal sulphates; (d) reacting the metal sulphates solution with a sourceof nitrate ions and of cations of which the sulphate is of lowsolubility, so as to prepare a solution of the nitrates of iron andchromium and an insoluble sulphate precipitate; (e) separating theinsoluble sulphate precipitate from the solution of the metal nitrates;and then, (f) combining the solution of the nitrates of iron andchromium of step b with the solution of the metal nitrate of step e torecover a solution of the nitrates of iron and chromium from said spentcatalyst.
 3. A method according to claim 1 in which the said source ofnitrate ions in step b is calcium nitrate, and the precipitated sulfateis calcium sulphate.
 4. A method according to claim 2 in which the saidsource of nitrate ions in step d is calcium nitrate.
 5. A methodaccording to claim 3 which comprises, after separation of calciumsulphate, reacting the nitrate solution with a source of cations forminga sulphate less soluble than calcium sulphate.
 6. A method according toclaim 4 which comprises, after separation of calcium sulphate, reactingthe nitrate solution with a source of cations forming a sulphate lesssoluble than calcium sulphate.
 7. A method according to claim 1 or claim2 in which a ferrous compound or metallic iron is also dissolved in thesulphuric acid, and a source of hexavalent chromium is added to thesulphuric acid solution to maintain the ratio of iron to chromium in thesolution in the range of 20:1 to 6:1 calculated by weight as thesesquioxides.